1. Don't start with a generic question, like "What would you do if everything you believed in disappeared?"
2. Research the agencies before hand. Most of them have websites and will specifically list what they are and are not looking for. It also helps to include how you found them and what other books of theirs caught your interest.
3. Work on making a good hook (or intro sentence). Get a fresh set of eyes to look over it. Work on it again. That first sentence is often the most difficult.
4. Study jacket flaps of novels you enjoy to see how they hooked you in a few hundred words or less. Using specific descriptions helps. For example, I initially described my protagonists's life as "strict" and "planned". When I took that out and instead mentioned her "compulsive list making", it made for a tighter sentence.
5. Don't include secondary characters and plotlines! Only your protagonist should be saving the day.
6. Include the word count and specific genre.
P.S. I'll be resuming regular writing and non-writing posts tomorrow! Sorry I was absent the past week. Finals and other projects kept me caffeinated and cranky.